Powdered alloys of main group elements and conductive powders such as carbon black have been used to make electrodes for lithium-ion cells in a process that involves mixing the powdered active ingredients with a polymeric binder such as polyvinylidene fluoride. The mixed ingredients are prepared as a dispersion in a solvent for the polymeric binder, and coated onto a metal foil substrate, or current collector. The resulting composite electrode contains the powdered active ingredient in the binder, adhered to the metal substrate.
Many polymers such as polyvinylidene fluoride, aromatic and aliphatic polyimides and polyacrylates have been used as binders for metal and graphite based lithium-ion cell electrodes. However, the first cycle irreversible capacity loss in the resulting cells can be unacceptably large, e.g., as large as 300 mAh/g or more for an electrode based on a powdered metal material.
Secondary electrochemical cells, such as lithium-ion cells, are capable of being reversibly charged and discharged multiple times. In the case of lithium-ion batteries, the charging and the discharging of the lithium-ion electrochemical cells are accomplished by lithiating and delithiating the cell electrodes. When lithium-ion cells are constructed, they usually contain excess lithium-ions in the positive electrode and no lithium-ions in the negative electrode. During the initial cycling reaction of the cells (charging), lithium transfers from the positive electrode to the negative electrode until the negative electrode has reached its capacity of absorbing lithium-ions. Upon the first discharge, the lithium-ions migrate from the lithiated negative electrode back to the positive electrode. Typically, after the first charging not all of the lithium-ions in the negative electrode are able to migrate out of the negative electrode. This results in what is known as irreversible loss in the capacity of the cell. Loss in the capacity of a cell from additional cycling (after the first cycle) is called capacity fade. This can be for a variety of reasons including changes in the morphology of the active electrode material upon repeated cycling, a buildup of insulating layers on the active electrode material upon repeated cycling or other reasons. A desirable lithium-ion cell is one that has low irreversible capacity loss after the initial cycling, and has low capacity loss (fade) after multiple cycles.